The invention described herein generally relates to an apparatus for detecting the presence of rock during coal mining operations, and more particularly, to an armored detector system, utilizing sensitive monitoring equipment, such as radiation detecting equipment, which is used in mining operations to allow removal of essentially all the coal with very little cutting into the rock above and below the coal.
The use of sensitive monitoring equipment in mining operations is well known. It is further known that radiation sensors in particular are well suited for use in coal mining operations. Their conventional use allows for limited control of the cutting depth for a variety of continuous excavators used in mining operations. However, effective use of gamma detectors has been impaired due to the inability to place the detectors such that they can accurately measure the thickness of the coal remaining to be cut or, in effect, to accurately measure the distance between the cutter and the rock that is to be avoided. Conventionally, suitably sized detectors have only been able to make real-time measurements at locations other than in the region actively being cut and then have inferred or calculated, in a somewhat indirect manner, the parameter that ultimately must be known; namely, the distance from the cutter to the rock. Further, such conventional approaches have tried to project cutting decisions to future or succeeding cuts rather than making real time cutting decisions during the current cutting stroke. Such approaches have only had limited success, particularly on continuous miners, because of the large variations in the formations, cutting conditions and other operational variables.
In coal mining operations, radiation sensors, such as gamma sensors, are currently used to detect radiation emissions from layers of fireclay and shale and other non-coal materials in the surrounding ground. Radiation is emitted from non-coal layers in various quantities dependent upon the type of non-coal material. As the radiation passes through the coal from the rock, it is attenuated. It is this attenuation that is measured, or counted, to determine when cutting should be halted to avoid cutting into the rock. Counting gamma rays must be accomplished over a period of time because the nature of radiation is statistical, having an emission rate that is represented by a Gaussian distribution around some central value.
The most accurate measurements of the distance from the cutter to the rock to be avoided is to place the sensor near the region of the mineral being cut, rather than at a distance away or near some other region. Data must be accumulated over time in order to average the readings so as to establish that central value. Since the radiation in a coal mines is relatively weak, the view angle needs to be large in order to obtain data in a sufficiently short time in order to be used to control real-time cutting actions. But, large view angles in conventional devices have resulted in viewing radiation sources other than from the region that needs to be measured so this makes the measurement inaccurate. In other words, choosing a narrow viewing angle has reduced the count rate, requiring more time which resulted in decreasing the accuracy since the miner is active and must continue. But, making the view angle wider also has reduced the accuracy.
It is also known that radiation detecting equipment is sensitive and must be protected from harsh environments to survive and to produce accurate, noise free signals. This protection must include protection from physical shock and stress, including force, vibration, and abrasion, encountered during mining operations. However, the closer in proximity equipment is to the mineral being mined, the greater is the shock, vibration and stress to which the equipment is subjected. Thus, there is a tension between placing conventional radiation detectors close to the surface being mined to make accurate measurements and providing adequate protection to ensure survival of the sensor and to avoid degradation of the data by the effects of the harsh environment. Conventionally, the need to assure survival of the sensor has resulted in placement of the sensor away from the target of interest. Another conventional approach has been to make the sensing element smaller so that it can be more easily placed in a strategically desirable location, but the sensitivity of the element drops as the size is reduced, and again, the accuracy reduces in a corresponding fashion.
It is important for ensuring reliable data that excess noise and/or degradation of data due from shock be reduced. To optimize the efficiency of the transmission of data from a scintillation element to a photomultiplier tube, it is known to place an optical coupling between the element and the tube. The optical coupling may entail applying optical grease to a window for the scintillation element and a faceplate of the photomultiplier tube and pressing the window and faceplate together. Such interfaces are unreliable under high vibration and shock and degrade over time as the grease tends to migrate from the interface.
Another optical coupling is directly bonding the photomultiplier tube faceplate to the window or to the scintillation element itself. While such an interface is generally of good quality, it requires special skills and equipment to perform the bond properly. Further, such a bond does not allow easy separation or replacement (especially within an explosion-proof housing) and it dynamically connects the photomultiplier tube and the scintillation element together.
Yet another optical coupling is placing an elastomeric transparent disk between the photomultiplier tube and the scintillation element with grease on either side. Disadvantages to this optical coupling include that the grease tends to migrate from the interfaces, changing the optical coupling properties, and that noise may be created. Further, in some configurations, such an optical coupling is difficult to install and retain.
Instead of smooth surfaces, some optical coupler disks have oil retaining rings, such as described in U.S. Pat. No. 5,962,855 (Frederick et al.). Such optical coupler disks have disadvantages when the photomultiplier tube is installed into an explosion-proof housing, since absolute precision regarding the placement of the optical coupler disk between the photomultiplier tube and the scintillation element is essential.
One method of mining coal is continuous mining, in which tunnels are bored through the earth with a machine including a cutting drum attached to a movable boom. The operator of a continuous mining machine must control the mining machine with an obstructed view of the coal being mined. This is because the operator is situated a distance from the cutting made by the picks on the cutting drum and his v iew is obstructed by the portions of the mining machine as well as dust created in the mining operation and water sprays provided by the miner. Another method of mining coal is longwall mining, which also involves the use of a cutting drum attached to a boom. In longwall mining, as compared with continuous mining, the drum cuts a swath of earth up to one thousand feet at a time. Both continuous mining machines and longwall mining machines are used in very harsh conditions.
Space for installing a gamma detector on a continuous miner is very limited since the detector must be positioned in a specific location in order to be in view of the coal to rock interface. The presence of armor, which is required to protect the detector, further limits the available space. An explosion-proof housing takes up even more of the available space, and often results in reducing the diameter of the photomultiplier tube. As the diameter of the photomultiplier tube is reduced, the efficient transfer of light to the tube becomes more critical. The optical coupling thus must be as thin as possible while remaining durable.
The invention provides a photomultiplier apparatus for use with a gamma detector which includes a photomultiplier tube, a faceplate located on an end of the photomultiplier tube, and an optical coupler molded to the faceplate.
The invention also provides a gamma detector that includes a scintillation element and the photomultiplier apparatus.
The invention also provides a method of molding an optical coupler directly to a photomultiplier tube. The method includes placing the photomultiplier tube within an optical coupler molding fixture. The fixture includes a frame with a frame base, a clamping structure, a shim, and a mold. The method further includes the steps of abutting one end of the photomultiplier tube against the shim, centering the photomultiplier tube within the frame, clamping the mold onto the shim, injecting an optical material into the mold, and curing the material.
The invention further provides an optical coupler molding fixture for molding an optical coupler onto a photomultiplier tube. The fixture includes a frame with a frame base, the frame being adapted to receive a photomultiplier tube, a shim, a mold, and a clamping structure for clamping the frame base and the mold toward said shim.
These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention which is provided in connection with the accompanying drawings.